Image or optical scanners are enjoying ever-growing utility in a variety of products and for a diversity of applications. For the purpose of understanding the subject invention "image or optical scanners" are defined as including of one or more photoresponsive circuits or elements disposed to optically scan a pattern of data and produce a signal representative thereof.
Optical scanners may be readily adapted to address a wide variety of data inputs. The data may be in the form of a photograph, a drawing, a design on fabric or the like or any other such graphic patterns. In other instances, the data being scanned may be alpha-numeric data such as printed or written matter. Basically, image scanners convert a pattern of data into an electrical signal which may be supplied to downstream apparatus for further processing, storage or display. Image scanners are incorporated into facsimile machines, copying machines, computer input terminals, CAD-CAM systems and the like. Additionally, image scanners are utilized in production processes to inspect surfaces of materials such as plywood, fabric, and metal. The typical image scanner includes one or more photoresponsive circuits disposed so as to either simultaneously, or sequentially address various portions of the surface being scanned.
There are several approaches currently employed for the fabrication of image scanners. Charge coupled devices (CCD's) form the basis for one such approach. CCD's are solid state devices, typically formed from crystalline silicon and including therein a plurality of photoresponsive circuits each having a pixel associated therewith. CCD's have a high degree of photosensitivity and are capable of providing high resolution. However, CCD's are relatively small in size; the typical CCD array is approximately one inch in length, and the largest CCD's currently produced are approximatey 3 to 4 inches in length. These size constraints impose restrictions on the utility of CCD's in scanners. In those instances where a pattern of information having dimensions larger than that of the CCD is being scanned, an optical system must be utilized to project that pattern of information at a reduced size onto the surface of the CCD. Aside from being expensive and bulky, such optical systems will effectively reduce the resolution of the CCD.
Thin film devices represent another approach to the fabrication of image scanners. Thin film devices may be formed by vapor deposition of layers of appropriate semiconductor materials onto a variety of substrates. By appropriately patterning these layers, a variety of device configurations may be fabricated.
Recently, considerable progress has been made in developing processes for depositing thin film semiconductor materials. Such materials can be deposited to cover relatively large areas and can be doped to form p-type and n-type semiconductor materials for the production of semiconductor devices such as p-i-n type photodiodes equivalent, and in some cases superior to those produced by their crystalline counterparts. One particularly promising group of thin film materials are the amorphous materials. As used herein, the term "amorphous" includes all materials or alloys which have long range disorder although they may have short or intermediate range order, or even contain at times, crystalline inclusions. Also as used herein, the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, band gap and absorption constant occur.
It is now possible to prepare by glow discharge, or other vapor deposition processes, thin film amorphous silicon, germanium or silicon-germanium alloys in large areas, said alloys possessing low concentrations of localized states in the energy gap thereof and high quality electronic properties. Techniques for the preparation of such alloys are fully described in U.S. Pat. Nos. 4,226,898 and 4,217,374 of Stanford R. Ovshinsky, et al., both of which are entitled "Amorphous Semiconductor Equivalent to Crystalline Semiconductors" and in U.S. Pat. Nos. 4,504,518 and 4,517,223 of Stanford R. Ovshinsky, et al., both of which are entitled "Method of Making Amorphous Semiconductor Alloys and Devices Using Microwave Energy"; the disclosures of all of the foregoing patents are incorporated herein by reference.
Thin film alloys may be readily manufactured in large areas by mass production processes and therefore enable the economic manufacture of large scale image sensor arrays. Use of such large arrays eliminates the need for complicated optical systems thereby effecting savings in cost, product size and processing steps. Additionally, since the thin film sensor arrays are fabricated to be of approximately the same size as the object being scanned, relatively high resolution may be attained without the necessity of employing high resolution photolithographic processing steps. It may thus be seen that thin film photosensor arrays have significant utility in the fabrication of image scanners.
Typical thin film image scanners include an array of photoresponsive circuits, each of which incorporate therein a photogenerative element adapted to provide an electrical signal corresponding to the quantity of light incident thereupon. It would be very time consuming to utilize a single element for scanning, accordingly, an array of elements in either linear or two dimensional form is typically utilized. In those instances where such an array is employed, each photosensitive circuit of the array must also include a blocking element such as a diode or transistor. The blocking element facilitates addressing of the various photogenerative elements in the matrix by preventing current flows through unwanted paths in the matrix. In this manner, the blocking device eliminates cross talk which would otherwise degrade the signal produced by the photosensitive element.
Problems occurred in the use of prior art photosensitive arrays because of the generation of charge carrier pairs within the blocking element thereof due to the absorption of incident illumination. Light having an energy greater than the band gap of the semiconductor material from which the blocking element is fabricated is capable of generating an electron-hole pair in that material. If a field is present across the semiconductor material, the photogenerated electron-hole pair is separated, thereby producing a flow of electrical current. Heretofore, such illumination of the blocking element was capable of producing a flow of electrical current which would effectively be a source of "noise" which dissipated or otherwise degraded the signal produced by and hence the sensitivity of the photogenerative elements. For this reason, it has heretofore been necessary to optically mask the blocking element so as to prevent the illumination thereof.
Such masking had been accomplished by depositing a relatively thick, opaque layer of metal or the like upon the blocking elements, or by utilizing a tape or other shield to prevent illumination. This approach is obviously less than satisfactory insofar as it necessitates extra processing steps. For example, depositing a metal layer requires great care if short circuiting is to be prevented. Use of a separately applied tape or other masking element necessitates disposing the blocking element relatively far from the photogenerative element so as to provide sufficient tolerances for placement of the masking member. Such remote placement wastes space which reduces resolution and furthermore requires the use of relatively lengthy connectors therebetween, which connectors add undesired capacitance to the circuit.
Accordingly, it can be seen that it would be highly desirable to eliminate the need for masking the blocking element from radiation incident thereupon insofar as such masking steps risk harm to the subjacent elements, increase cost, waste processing time, materials, and adversely increase the size and electrical capacitance of the photosensitive pixels.
The instant invention therefore provides for the fabrication of a photoresponsive circuit which includes a photogenerative element and a blocking element and in which there is no need for masking the blocking element from incident radiation.
These and other advantages of the present invention will be readily apparent from the summary of the invention, the drawings, the description of the drawings and the claims which follow.